Manatees are large, fully aquatic, marine mammals sometimes known as sea cows.
There are three accepted living species of Manatees:
• the Amazonian manatee (Trichechus inunguis)
• the West Indian manatee (Trichechus manatus)
• the West African manatee (Trichechus senegalensis)
Manatees inhabit the shallow, marshy coastal areas and rivers of the Caribbean Sea and the Gulf of Mexico ( West Indian manatee), the Amazon basin (Amazonian manatee), and West Africa (West African manatee).
Manatees are long-lived and reproduce slowly. Age estimates based on growth rings in ear bones indicate life spans of up to 60 years, and at least one manatee has survived more than 70 years in captivity.
Manatees weigh 400 to 550 kilograms (880 to 1,210 lb), and average 2.8 to 3 meters (9.2 to 9.8 feet) in length, sometimes growing to 4.6 meters (15 feet) and 1,775 kilograms (3,913 pounds). The females tend to be larger and heavier.
Dull gray, blackish, or brown in color, all three manatee species have stout tapered bodies ending in a flat rounded tail used for forward propulsion.
The forelimbs are modified into flippers – the flippers are used by all species for swimming, turning, bottom walking, and manipulating food; there are no hind limbs.
Manatees are excellent swimmers, able to stay submerged for more than 15 minutes, but they are incapable of travelling on land.
Overall, manatees appear slow moving and cumbersome, but they are graceful and can be swift underwater. Manatees usually travel at about 8 kilometers per hour (5 miles per hour), but in a pinch they can pick up the pace to 24 kilometers per hour (15 miles per hour). They often linger near the surface of the water, where they can suffer collisions with speedboats and other vessels.
While a manatee’s eyes may appear small, their vision is acute. They are able to distinguish different sized objects, colors, and patterns.
While sensory systems of manatees have not been well studied, their large inner ear bones may indicate their underwater communication. The animals emit chirps, whistles, and squeaks to reach out to others.
Manatees are uniquely adapted for eating aquatic plants. The manatee’s large lips are prehensile and studded with specialized sensory bristles and hairs (vibrissae) for discriminating between and manipulating food plants.
Compared with the fish and krill eaten by other marine mammals, most aquatic plants are low in energy value and protein. Manatees must therefore eat large amounts of this bulky, low-energy food to satisfy their dietary requirements. An adult manatee will commonly eat about 10% to 15% of their body weight (about 50 kg 110 pounds) per day. To handle such a diet, manatees are hindgut digesters (like horses) and have intestines as long as 30 meters (100 feet).
The teeth have also evolved in response to dietary demands. To counter abrasion from ingested sand and silica, manatees constantly grow new molars. These teeth progress from the rear of the jaws forward as older, worn teeth drop out at the front of the mouth. Unlike almost all other mammals, tooth replacement occurs throughout life.
Manatee brains are smooth (other animals, including humans have cortical folds) and its ratio of brain size to body size is the lowest of any mammal.
Manatees are active day and night and can sleep submerged or while breathing at the water’s surface.
They are primarily solitary but form small transient groups for periods of hours or days. Aggregations of up to 20 males may form mating herds centred on a receptive female; other temporary aggregations may form at feeding areas, freshwater seeps, or warm water sources. During extreme cold spells in Florida, aggregations of 300 or more have been observed in the warm water outflows of power plants.
Manatees typically breed once every two years; generally only a single calf is born. Gestation lasts about 12 months and to wean the calf takes a further 12 to 18 months.
The International Union for Conservation of Nature (IUCN) Red List of Threatened Species lists all three manatee species as Vulnerable and decreasing in numbers.
Manatees were traditionally hunted by indigenous Caribbean people. When Christopher Columbus arrived in the region, hunting was already an established trade, although this is less common today.
The closest living relatives to the manatee are the elephant and the hyrax. They diverged from a common land mammal over 50 million years ago.
The Order Sirenia consists of the three manatee species and another rare marine mammal called the dugong.